1. Field of the Invention
This invention relates to an electrode pattern forming method, and in particular, to a method which is applicable to a liquid crystal display device, an electroluminescence display device, a contact type image sensor which is composed of a photodiode array, etc. The method is used when forming that portion of these devices where transparent electrodes and Al electrodes are stacked on each other. This electrode pattern forming method protects the lower, transparent electrodes from corrosion when processing the Al electrodes by photoetching or the like.
2. Description of the Related Art
Transparent electrodes are used in display devices such as liquid crystal display devices (LCD) and electroluminescence display devices (ELD), and in electronics devices such as image sensors composed of photodiodes and/or photodiode arrays. In, for example, one electrode pattern forming method which is commonly used, an transparent conductive film is used, which is prepared by adding SnO.sub.2 to the main constituent In.sub.2 O.sub.3, conventionally known as indium-tin-oxide (ITO). An Al film with a low electrical resistance is formed on this ITO transparent conductive film for the purpose of providing wiring.
Normally, such an electrode pattern is formed as follows: first, a transparent film is formed on a substrate. Then, an Al film is formed on this transparent conductive film by evaporation, sputtering or the like. The Al film thus formed is processed into a predetermined electrode pattern by photoetching.
A resist material is used when forming an electrode pattern by photoetching; resist materials are of two types: negative and positive. In cases where high accuracy is required, positive type resist materials are preferred since they provide better resolution. When developing a positive type resist material, an alkaline developer is normally used. This, however, involves a problem, which will be described below with reference to FIG. 5(a). Here, an Al film 5 is formed on ITO electrodes 4, which are formed on a substrate 3, for the purpose of forming Al electrodes by photoetching. If a positive type resist material is used in this process, the ITO electrodes 4 can be corroded by the electrolyte 1, which serves as the developer for the resist material 6 on the Al film 5. Conventionally, this corrosion has been avoided by appropriately adjusting the developing time. That is, the time it takes for corrosion to take place is ascertained empirically and is taken into account when adjusting the developing time.
The corrosion of the ITO electrodes takes place as follows: let us imagine, as shown in FIG. 5(b), that local active corrosion points 11 have come into existence on the surface of the Al film 5. These active corrosion points 11 may be attributable to impurity segregation, distortion, or lattice defects. When this Al film 5 is immersed in the developer, i.e., the electrolyte 1, dissolution of Al occurs at the active corrosion points 11, with the result that microscopic pinholes 13 are generated in the Al film 5. Thus, a local cell is formed by the Al film 5, the ITO electrodes 4 and the electrolyte 1. In the corroded portions 12 of the ITO electrodes 4, In.sub.2 O.sub.3, which is the main constituent of ITO, is metallized through reduction and acts as one electrode of the above-mentioned cell. Thus, the rate of corrosion depends to a large degree on the thickness of the Al film or the quality thereof, or the quality of the ITO film. When forming an ITO film, an Al film or the like on a substrate, it is a more or less common phenomenon that the thickness and quality of the film vary depending on its position on the substrate. The thickness and quality of the film also change as the film formation is repeated.
Accordingly, the rate at which the ITO film is corroded is subject to changes, so that, with the conventional method, in which protection from corrosion is attained by adjusting developing time, it is difficult to avoid corrosion entirely. This is particularly true when the electrode formation area is relatively large. In addition, the degree to which the corrosion is avoided differs from substrate to substrate, thus making it difficult to increase the yield.